input size depending on input - c++

Question

If it makes sense.
This is how it looks so far.
#include <iostream>
int main()
{
char *buffer;
buffer = new char[100];
std::cin >> buffer;
const int size = strlen(buffer);
char input[size];
delete buffer;
return 0;
}
I know I can use the string library but I'm trying to do without it.
I want to make the char size (in the code char input) depending on the input size.
The errors which I am getting is
expression did not evaluate to a constant
expression must have a constant value
on line 12 which is the
char input[size];

Answer 1

I know I can use the string library but I'm trying to do without it.
You can use std::vector<char>. If that is not an option either, allocate memory yourself and make sure that you deallocate the memory.
char* input = new char[size+1]; // Add an extra for the terminating null characterr
....
delete [] input;

Answer 2

For std::cin you need to pre-allocate a buffer. If you plan on using char[] and you really have to avoid strings you have to make sure you allocate enough memory.
Alternatively you can read char by char with scanf("%c",&newChar) until user inputs an escape character and allocate memory for array as you go.
I don't suppose you're limited to using iostream ?

Answer 3

If it is the length of used space in the buffer array you are trying to assign to size for the input array, there are two methods you can use...
#include <iostream>
using namespace std;
int main()
{
char *buffer = new char[100];
cin >> buffer;
int spaceUsed = 0;
for (unsigned int i = 0; i < 100; i++)
{
if (buffer[i] != '\0')
spaceUsed++;
}
char input[spaceUsed];
delete [] buffer;
return 0;
}
OR...
#include <iostream>
using namespace std;
int main()
{
char *buffer = new char[100];
cin >> buffer;
int spaceUsed = 0;
while (*buffer++)
spaceUsed++;
char input[spaceUsed];
delete [] buffer;
return 0;
}
Both code snippets do the exact same thing. Of course, using the string library would make your life much easier though.